Take a few steps back.
Text-only emails do NOT show images, so you don't want to put it into your text-only emails. So editing create_account.php would be pointless.
HTML-formatted emails CAN show images, so you'd be better off putting the raw HTML for your IMG tag into whichever email templates you wish it to appear within. See the /email/ folder for those templates.
And of course, that then requires that your site be enabled to send emails in HTML format for those customers who have opted in to that. Admin->Configuration->Email Options->Send MIME HTML emails -- set to true.
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